1. Field of the Invention
The present invention relates to a control device for an internal combustion engine, for controlling an internal combustion engine provided with a supercharger which includes a turbine provided to an exhaust path and a compressor provided to an intake path.
2. Description of the Related Art
Conventionally, there is known a turbocharger including a supercharger to be mounted to an intake path of an internal combustion engine (hereinafter, referred to simply as “engine”), which is achieved for the purpose of improving an output of the engine. The supercharger moves a turbine by rotating the turbine with an exhaust gas. The turbocharger has a risk of damaging the engine due to a supercharging pressure which becomes higher than needed when the turbine is rotated at a high rpm under a high load. Therefore, in general, an exhaust-gas bypass path is provided at upstream of the turbine so as to divert a part of the exhaust gas flowing through an exhaust path into the exhaust-gas bypass path by a wastegate valve provided to the exhaust-gas bypass path, thereby adjusting the amount of the exhaust gas flowing into the turbine. In this manner, the supercharging pressure is controlled at an appropriate level (for example, see Japanese Patent Application Laid-open No. 9-228848).
Specifically, an exhaust pressure and the supercharging pressure of the supercharger are controlled by an opening degree of the wastegate valve. A control amount of the opening degree of the wastegate valve is determined by closed-loop control for a predetermined target amount of an intake system (for example, a set supercharging pressure or a set intake amount) based on an engine rpm and an engine load or simple open-loop control.
In recent years, the following control device for an internal combustion engine has been proposed. The control device provides good running performance by determining an air amount, a fuel amount, and ignition timing, which correspond to engine control amounts, using an output-shaft torque of the engine, corresponding to a requested value of a driving force by a driver or a vehicle, as a target engine output value.
Further, it is generally known that the control amount which most affects the engine output-shaft torque among all the engine control amounts is the air amount. In this context, a control device for an internal combustion engine, which controls the air amount with high accuracy, has also been proposed (for example, see Japanese Patent Application Laid-open No. 2009-013922).
The conventional wastegate-valve control device described in Japanese Patent Application Laid-open No. 9-228848 controls the opening degree of the wastegate valve by a target amount obtained by a map value previously determined in accordance with each of operation variables. Therefore, if there is: (1) an individual variability, (2) a change with time (deposit accumulation, deterioration of a bearing, or the like), (3) a change in viscosity of an engine oil (change in bearing loss), or (4) an environmental change (a turbine output, a turbine rpm, and a requested compressor driving force differ depending on an atmospheric pressure and a temperature) as a variation factor of the turbocharger, the control device is affected by a variation in control. Therefore, there arises a problem in that an actual operation point greatly deviates from a set operation point.
Moreover, the supercharging pressure is controlled by the opening degree of the wastegate valve. However, the supercharging pressure can also be controlled by an opening degree of a throttle valve. Therefore, for controlling the same supercharging pressure, there exist a plurality of combinations of the opening degree of the wastegate valve and the opening degree of the throttle valve. Thus, there is another problem in that an operation point, at which the optimal fuel efficiency is obtained, cannot be known.
Further, when the conventional wastegate-valve control device described in Japanese Patent Application No. 9-228848 is replaced by the control device for the internal combustion engine, described in Japanese Patent Application No. 2009-013922, which uses the engine output as the target value, the control is also replaced by control for the engine output with respect to an opening degree of an accelerator, that is, control for drivability (=acceleration). Therefore, a plurality of map values are required to be set in accordance with the opening degree of the accelerator as the target amounts of the opening degree of the wastegate valve. Therefore, the operation point also changes. Thus, even in this case, there is a problem in that the setting of the ignition timing and an air/fuel ratio is required to be changed.